A light emitting diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from active layers of semiconductor material sandwiched between oppositely doped layers when a voltage is applied across the doped layers. Multiple LED dies are typically packages together in an LED array. In one example, an array of LED dies is mounted onto a heat conducting substrate, and a layer of silicone is disposed over the LED dies. When a current is driven through the LED dies, the dies emit blue light. Some of the blue light emitted from the LED dies is absorbed by phosphor particles embedded in the silicone and is re-emitted by the phosphor particles as longer wavelength light. The combined light emitted by the LED dies and phosphor particles has a wider band of wavelengths. Some of the emitted light does not, however, exit the LED package because the light is absorbed by surfaces of the package. Thus, a first desirable quality of LED packaging is having highly reflective surfaces that reflect as much light as possible from the LED package.
In addition, while emitting light, the LED dies and the phosphor particles also generate heat. The performance and operational lifetime of the LED dies is degraded if the operating temperature exceeds a threshold level. Empirical data demonstrates that there is an inverse relationship between the useful life of an LED die and the amount by which the average operating temperature exceeds a threshold level, such as 25 degrees Celsius. In order to remove enough heat from the LED dies so as to keep the LED dies adequately cool, the LED package is typically fixed in some way to a heat sink. Thus, a second desirable quality of LED packaging is effectively dissipating the heat generated by the LED dies.
And of course low cost is a third desirable quality of LED packaging. In an attempt to reduce the cost of LED packaging, bare LED dies can be attached directly to a large printed circuit board in a chip-on-board (COB) manner in order to eliminate the manufacturing step of mounting the dies onto smaller substrates that are themselves attached to the board. The overall cost can be reduced by transferring the manufacturing steps of die attaching, wire connecting and encapsulating to the circuit board assembly stage.
FIG. 1 (prior art) is a cross-sectional view of chip-on-board packaging 10 in which light-emitting diode (LED) dies 11-16 are directly mounted onto an aluminum substrate 17 that is covered by a reflective layer 18 and an insulating layer 19. Conductive traces 20 are deposited over the insulating layer 19, and bond wires 21 electrically connect contacts on the LED dies 11, 16 to the conductive traces 20. Subsequently, silicone 22 containing phosphor particles 23 is poured into a dam 24 and hardens forming a conformal covering over the LED dies 11-16. The silicone layer 22 also protects the bond wires 25. Although the chip-on-board packaging 10 does exhibit the two desired LED packaging qualities of effective heat dissipation and high reflectivity, the cost of using the highly reflective aluminum substrate as an entire surface of the LED luminaire is exceedingly expensive.
A low cost method of packaging LED dies is sought that provides superior reflectivity and thermal conductivity but yet reduces cost by avoiding covering entire surfaces of the LED luminaire with expensive highly reflective layers.